It is known to reduce the temperature difference between the head of the valve and the head of the cylinder--into which the valve seat ring is worked--by heat transport in the valve. The shaft of the valve is provided for said purpose with a hollow bore and is cooled. The diameters of valve shafts have been reduced in the last few years for cost and weight reasons in such a way that it is no longer possible in most cases to provide such shafts with a hollow bore, so that the application of valves drilled hollow and filled, for example with sodium, will no longer be possible in the future. Therefore, efforts are being made for improving the thermal conductivity of the material from which the valve seat and in particular the valve seat ring is manufactured, in order to discharge heat in this way more rapidly and to lower the temperature level for the purpose of enhancing the tribological conditions and the system both technologically and in terms of cost.
Powder-metallurgically manufactured shaped articles are known which are produced from sintered materials based on iron with infiltrated copper. Such materials are sufficiently wear-resistant to be employed for manufacturing valve seat rings or valve guides; however, the thermal conductivity of such materials is not high enough as compared to sintered materials without copper component. For example, a sintered material is known from DE-PS 21 14 160, which consists of an iron base material, to which carbon and lead as well as other alloying components are added. Valve seat rings produced from said material do have adequate resistance to heat and wear; however, their thermal conductivity is inadequate for solving the problem here on hand especially within the region of the outlet of a modern internal combustion engine.
A sintered material for the powder-metallurgical production of valve seat rinds is known from PCT-EP 89/01343. Such valve seat rings are expected to have increased thermal conductivity combined with high resistance to wear. The sintered material consists of a basic metal powder with a copper component of about 70% to 100% by weight, as well as with an alloying component. The latter may consist of, for example 1 to 3% by weight cobalt or a highly alloyed additional metal powder added to the basic metal powder as a hard phase, the proportion of which then comes to 30% by weight at the most.
Tests carried out with such a material have shown that the material has a resistance to wear which is not sufficient for the manufacture of valve seat rings, and particularly not for the outlet region of internal combustion engines. This has to be attributed to the fact that even though it was possible to increase the hardness of the material through solidification of the matrix by incorporating hard substances with a maximum particle size of 150 .mu.m, and thus to increase the resistance of the valve seat ring to wear, on the one hand, the counter body showed stronger wear due to the relatively large and sharp-edged incorporated particles, on the other. Therefore, the wear on the valve seat ring was low, whereas the overall wear, which is important to the lasting functioning of the system, became worse.